Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-12-06
2001-07-17
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S296000, C250S492200
Reexamination Certificate
active
06261726
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to masks used in photolithography of integrated circuit designs, and in particular, masks used in ray or charged particle beam lithography.
2. Description of Related Art
One of the most critical parameters of masks fabricated for advanced photolithography of integrated circuit designs is image placement error. This is particularly important for membrane masks such as those used in x-ray or charged particle beam lithography such as ion beam and, in particular, projection electron-beam lithography (PEBL). An embodiment of PEBL is referred to as the SCALPEL™ (scattering angular limited projection electron lithography) process. Projection electron-beam lithography requires masks that are at least semi-transparent to electrons. For this reason, these masks contain membranes that are very thin, on the order of 50-200 nm.
An example of the well-known PEBL lithography is shown in FIG.
1
. Electron beam radiation
30
passes through a portion of mask
18
comprising a membrane layer
20
on which are deposited scatterer layer segments conforming to the circuit image to be projected. The electron beam
30
portion that passes with relatively low scatter through the membrane
20
portion between scatterer segments
22
is illustrated by beam
30
a
. Beam
30
a
is focused by lens system
24
and through an opening in back focal plane filter
26
onto the surface
32
of a semiconductor wafer having conventional resist materials sensitive to the electron beam radiation. Electron beam portion
30
b
which is scattered to a greater degree by scatterer segments
22
is somewhat focused by lens
24
but does not pass fully through the opening in filter
26
. Thus, a contrasting image
28
is formed on wafer resist surface
32
which conforms to the pattern of scatterer layer
22
on the mask.
These lithography masks have to be robust, resistant to breakage and strong enough to minimize any in-plane distorting forces that occur as a result of processing. To minimize membrane breakage, the mask structure includes supporting struts which subdivide the membrane into areas of equal size and aspect ratio. It is within these uniform membrane areas that the radiation-absorbing scatterer or patterning layers must be formed to project the configuration of the individual circuit features or elements onto the different chip layers.
The format for a typical PEBL or SCALPEL™ mask is shown in
FIGS. 2 and 3
. The mask consists of a patterned scatterer layer
22
on top of a thin membrane material
16
of thickness t. In the side view shown in
FIG. 2
, membrane
16
covers essentially all the upper surface of mask
18
. Typical scatterer layer materials include W, Cr/W, TaSi, Cr/TaSi, and combinations with other Ta-based compounds. Because the membrane is so thin, silicon struts are needed to reinforce the membrane, and strengthen the mask structure. The underlying support is formed from silicon layer
34
and protective layer
36
, and openings are etched therein to form equi-spaced supporting struts
42
in a uniform pattern within outer supports
44
. On a typical PEBL mask, there will be many membranes interlaced with struts in an arrangement similar to the one shown in FIG.
2
. It is between struts
42
that the individual scatterer layers form patterns
22
corresponding to circuit features or elements on membrane areas
20
. Each membrane area
20
needs to be small enough so that it is self-supporting, and any distortions due to patterning the scatterer layer are minimized. As shown in the bottom view in
FIG. 3
, the membrane areas
20
in the openings between struts
42
which contain the mask features have uniform size and aspect ratio. A typical size of the discrete membrane areas over the entire mask is about 12 mm
2
with a typical aspect ratio of 12:1 (width:length) as seen in plan view, or about 1 mm
2
with an aspect ratio of 1:1, again over the entire mask.
Alternatively, in stencil masks used for ion beam lithography or other configuration of electron beam lithography, the membrane material is used as the radiation absorber and openings which permit radiation passage are made which conform to the configuration of the individual circuit elements. In either case, the circuit design is subject to the size and aspect ratio of the uniform membrane areas on the mask.
Typical prior art membrane thicknesses and discrete membrane areas for various photolithography mask technologies are shown below in Table 1:
TABLE 1
Discrete
Membrane
Membrane
Thickness
Area
Area/Thickness
Mask Technology
(nm)
(mm
2
)
(mm
2
m)
SCALPEL ™
150
13
0.09
PEBL stencil
1500
1
0.001
IPL stencil
3,000
2,500
0.83
X-ray
2,000
750
0.38
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved mask for use in ion beam and/or projection electron beam lithography.
It is another object of the present invention to provide such a lithography mask which is stronger and more robust than prior art masks.
A further object of the invention is to provide such a lithography mask which may be used for complex circuit patterns without limiting strength.
It is yet another object of the present invention to provide masks for the aforementioned uses which do not have prior restrictions on membrane opening size or aspect ratio.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
SUMMARY OF THE INVENTION
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a mask for photolithographic transfer of an image corresponding to an integrated circuit from the mask onto a semiconductor substrate comprising a patterned scattering or absorbing layer on a top surface of a partially energy transparent membrane in an inner region of the membrane; a peripheral support ring supporting an outer region of the membrane; and support struts supporting the inner region of the membrane, the struts connected to the support ring and aligned to major design elements of the integrated circuit. Preferably, the membrane is formed of a material having a Young's modulus of at least about 400 GPa, and the membrane comprises material selected from the group consisting of silicon carbide, diamond, diamond-like carbon, amorphous carbon, carbon nitride and boron nitride. The support struts form and surround a plurality of discrete membrane areas of different sizes and aspect ratios aligned to design regions of an integrated circuit.
In a related aspect, the present invention provides a photolithography mask comprising a membrane layer, preferably of a material having a Young's modulus of at least about 400 GPa, and support struts supporting a surface of the membrane. The struts form and surround a plurality of discrete membrane areas of different aspect ratios aligned to design regions of an integrated circuit. There is also provided means for providing a contrasting image within the discrete membrane areas. The contrasting image means is used to expose the mask in a lithography process in patterns and corresponding to design elements within the integrated circuit design regions. Preferably, the discrete membrane areas have different aspect ratios which range from about 1:1 to about 12:1, and the discrete membrane areas have different size surface areas. The discrete membrane areas are free of support other than the surrounding support struts.
The membrane preferably comprises a material selected from the group consisting of silicon carbide, diamond, diamond-like carbon, amorphous carbon, carbon nitride and boron nitride. In one embodiment, the contrasting image means comprises patterned layers on a surface of the membrane within the discrete membrane areas corresponding to design elements within the integrated circuit design reg
Brooks Cameron J.
Lercel Michael J.
Powers Lynn A.
DeLio & Peterson LLC
International Business Machines - Corporation
Kotulak Richard M.
Peterson Peter W.
Rosasco S.
LandOfFree
Projection electron-beam lithography masks using advanced... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Projection electron-beam lithography masks using advanced..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Projection electron-beam lithography masks using advanced... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2533388